This invention relates to a starch based corrugating adhesive composition containing soluble high amylose starch as the carrier starch. The high amylose starch is rendered soluble before it is added to the adhesive. The adhesive may be provided in the form of a "one piece" dry mixture and used without first cooking the carrier starch in alkali. The use of alkali or borax in the dry mix is optional, and the adhesive is pH independent.
This invention also relates to a process for manufacturing corrugated paperboard employing a high amylose starch based corrugating adhesive that is provided as a one piece dry mixture and prepared for use in corrugating by simply dispersing the dry mixture in water at 25.degree. to 55.degree. C.
The manufacture of corrugated paperboard usually involves a continuous process whereby a strip of paperboard is first corrugated by means of heated, fluted rolls. The protruding tips on one side of this fluted paperboard strip are then coated with an adhesive, and a flat sheet of paperboard, commonly known in the trade as a facing, is thereafter applied to these tips. By applying heat and pressure to the two paperboard strips, an adhesive bond is formed between the strips. This produces a single-faced board in that the facing is applied to only one surface. If a double-faced paperboard is desired, in which an inner fluted layer is sandwiched between two facings, a second operation is performed wherein the adhesive is applied to the exposed tips of the single-faced board and the adhesive-coated tips are then pressed against a second facing in the combining section of the corrugator under the influence of pressure and heat. The typical corrugating process and the use of corrugators in general are described in U.S. Pat. Nos. 2,102,937 and 2,051,025 to Bauer.
The particular adhesive employed in the corrugating process is selected on the basis of several factors, including the type of bond required in the box, package, or other item in which the finished corrugated product is to be used. Starch-based adhesives are most commonly used due to their desirable adhesive properties and low cost.
The most commonly used starch based corrugating adhesives is an alkaline adhesive which is comprised of raw, ungelatinized starch (raw starch) suspended in an aqueous dispersion of cooked starch (carrier starch). The adhesive is produced by gelatinizing starch in water with sodium hydroxide (or other alkali) to yield a primary mix of gelatinized or cooked carrier starch, which is then slowly added to a secondary mix of raw starch, borax and water to produce the full-formulation adhesive. In the corrugating process, the adhesive is applied (usually at between 25.degree. and 55.degree. C.) to the tips of the fluted paper medium or single-faced board, whereupon the application of heat causes the raw starch to gelatinize, resulting in an instantaneous increase in viscosity and formation of the adhesive bond.
There are different theories regarding the respective roles of the raw and the carrier starches in the development of adhesive properties, but there is substantial evidence to support the view that the carrier contributes to the bond strength and set speed of the adhesive, and that good tack in the carrier leads to good tack in the adhesive and, therefore, improved runnability in the full-formulation adhesive. (See R. Williams, C. Leake and M. Silano, TAPPI, Vol. 60 No. Apr. 4, 1977 , pp. 86-89.)
It has been known for many years that a corrugated adhesive whose carrier portion is prepared from a high amylose starch (i.e., at least 40% amylose) is superior to one prepared from pearl starch, which contains about 27% amylose, because a carrier can be produced having improved rheological and film-forming properties, and increased moisture resistance. There are strong indications that the tack of the carrier also plays an important role in the corrugating process. Furthermore, improvements in tack (green bond strength) permits higher corrugating machine speeds as compared to corrugating adhesives known in the prior art. Improved high amylose starch based corrugating adhesives are disclosed in U.S. Pat. No. 4,787,937, issued Nov. 29, 1988, to Leake, which is hereby incorporated by reference.
U.S. Pat. No. 3,487,033, issued Dec. 30, 1969, to D. E. McElmury, describes a process for producing a single starch component adhesive by reaction of the ungelatinized starch in alkali and water in specified proportions and under heat conditions to provide a suspension of starch containing primarily ungelatinized starch in partially swollen form. This process requires complex preparation steps by the manufacturer of corrugated paperboard. In the absence of a crosslinking agent (e.g., urea formaldehyde or a crosslinking resin), the adhesive prepared by this process does not provide water resistance.
Commercial corrugating operators typically purchase the raw and carrier starches, alkali and other adhesive components as separate items, then cook the carrier starch in alkali and water and, in a separate step, add the raw starch and other components to the cooked carrier starch to form the adhesive before running the corrugating operation. The alkali used in cooking the carrier starch advantageously lowers the starch gelatinization temperature, and is particularly advantageous when high amylose starch is employed as the carrier starch. High amylose starch requires a higher cooking temperature for gelatinization than other starches (e.g., in the absence of alkali, 135.degree. to 170.degree. C. is required for high amylose starch versus 70.degree. to 80.degree. C. to gelatinize other starches). Without sufficient alkali, the high amylose carrier starch cannot be gelatinized by the corrugating operator using an atmospheric pressure cook. Cooking at greater than atmospheric pressure is needed. Thus, the alkali cook has become a highly desirable component of the starch based corrugating adhesives known in the art, and an essential component of those adhesives comprising high amylose starch.
Smaller amounts of alkali are used as a component of conventional liquid adhesives that do not contain high amylose starch to optimize adhesive performance.
It has been discovered that by selection of certain soluble high amylose starches that have been pregelatinized by an extrusion process, a coupled jet-cooking/spray-drying process, a steam-injection/atomization spray-drying process, or other, similar processes, a pH independent, high amylose starch corrugating adhesive may be prepared and delivered as a one-piece dry mixture. These soluble high amylose starches also may be used separately to provide the carrier starch component of a corrugating adhesive without first cooking the starch in alkali. The carrier component can be prepared simply by dispersing the soluble high amylose starch in water at 25.degree. to 55.degree. C.
One-piece dry mixtures have been used commercially, however, high amylose starch has never been used in these mixtures. Until now, high amylose starch could not be used effectively in a one-piece corrugating adhesive because of the extreme cooking conditions needed to disperse the high amylose starch.
Thus, the adhesives of this invention offer the convenience of a dry mixture that can be dispersed in hot or cold water without cooking, independent of pH, together with the adhesive strength, water resistance, higher corrugator speed and set speed and other benefits associated with high amylose starch. Because these adhesives can be used without alkali, they exhibit better water resistance and do not require the use of a crosslinking agent to obtain a water resistant corrugated paperboard.